The present invention relates to a burner for use in connection with industrial furnaces which are capable of firing either gaseous or liquid fuels and of efficiently operating over a wide operating range with relatively low excess air and at low emission levels.
The increasingly serious fuel shortage requires that furnace, particularly for industrial applications, operate as efficiently as possible over the entire operating range of such burners. Frequently, the range may extend from a maximum capacity to as low as 5% thereof. In the past, improvements in the operating efficiencies of such burners were attempted by such diverse means as minimizing of the combustion air supply or by controlling the flame temperature, the flame shape and size, the combustion air speed past the fuel nozzles, etc. Hand in hand with the need for an improved operating efficiency goes the requirement for minimizing the generation and discharge of pollutants such as soot or nitrogen oxides (hereinafter "NOX").
In the past, the perhaps most noteworthy development in this regard has been the production of burners which have a centrally located fuel nozzle, a flame stabilizer or "swirl plate" positioned concentrically immediately upstream of the nozzle so as to induce turbulence to the combustion airflow in the vicinity of the nozzle, while an additional combustion airflow disposed radially outward of and surrounding the turbulent core flow remains substantially laminar. In this manner, excessive flame turbulence was reduced as contrasted with other prior art burners which employed flame stabilizers extending over the entire or at least over a substantial portion of the cross-section of the combustion airflow past the nozzle. Additionally, the combustion air conduit of the burner sometimes received an advantageous venturi configuration in the vicinity of the fuel nozzle which has a tendency to reduce the power requirements of the combustion air fan.
An article by H. P. Niepenberg entitled "Entwicklungstendenzen Im Grossbrennerbau" (Trends in the Development of Large Burners) appearing in the German magazine Technische Mitteilungen, 65th Year, Volumne 3 of March, 1972 traces the development of such burners and discusses their construction in some detail.
Although these prior art burners were no doubt a significant improvement over other and earlier burners, their operating efficiency still was not as high as one might hope for. At least in part, this appears to be a result of the relatively turbulent flame generated by such burners which requires a significant amount of excess air which, of course, lowered their operating efficiency. Further, the flame turbulence inceased the combustion speed and thereby increased the maximum temperatures developed in the flame which, in turn, has a tendency to increase the generation of polluting NOX. Although certain refinements and changes could be made to the burner, such as the provision of the above-mentioned venturi throat to enhance certain aspects of its operation, the overall efficiency and pollutant discharge volume could only be insignificantly changed.
Thus, there is presently a need for a burner of the type generally discussed above which departs from prior art burner configurations so as to both significantly enhance its operating efficiency while limiting or, preferably, substantially reducing the pollutants, especially NOX.